Bifacial solar cell

ABSTRACT

A bifacial solar cell includes a substrate; an emitter portion formed on a first surface of the substrate; a first insulating layer formed on the emitter portion; a plurality of first electrodes contacting the emitter portion through the first insulating layer and extended in a first direction; a plurality of first current collectors extended in a second direction crossing the first direction, wherein the plurality of first current collectors are electrically and physically connected to the plurality of first electrodes; a second insulating layer formed on a second surface of the substrate; a back surface field formed on the second surface of the substrate, and having an impurity concentration that is higher than an impurity concentration of the substrate; a plurality of second electrodes contacting the back surface field through the second insulating layer and extended in the first direction; and a plurality of second current collectors extended in the second direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of co-pending U.S. patent applicationSer. No. 13/212,919 filed on Aug. 18, 2011, which claims the benefitunder 35 U.S.C. § 119(a) to Korean Patent Application No.10-2011-0028041 filed on Mar. 29, 2011, all of which are herebyexpressly incorporated by reference into the present application.

BACKGROUND OF THE INVENTION

Field of the Invention

The embodiments of the invention are directed to a bifacial solar cell.

Discussion of the Related Art

Solar cells convert light into electric power using a photovoltaiceffect.

A solar cell includes a substrate and an emitter portion that form a PNjunction. Light is incident on a surface of the substrate to create acurrent.

In general solar cells, light is received only through one surface eachsolar cell, thus exhibiting low photovoltaic efficiency.

There is a need for a bifacial solar cell that receives light throughtwo opposite surfaces of a substrate.

SUMMARY OF THE INVENTION

According to an embodiment of the invention, there is provided abifacial solar cell including a substrate, a plurality of firstelectrodes provided on a first surface of the substrate in a firstdirection, a plurality of first current collectors provided on the firstsurface in a second direction crossing the first direction, wherein theplurality of first current collectors are electrically and physicallyconnected to the plurality of first electrodes, a plurality of secondelectrodes provided on a second surface of the substrate in the firstdirection, and a plurality of second current collectors provided on thesecond surface in the second direction, the plurality of second currentcollectors being electrically and physically connected to the pluralityof second electrodes, wherein at least one of the plurality of secondelectrodes is positioned on a different reference line from a referenceline of at least one of the plurality of first electrodes.

The solar cell creates current using light incident on the first andsecond surfaces of the substrate. Accordingly, the efficiency of thesolar cell is further improved compared to a solar cell that receiveslight only through a surface.

Further, among light beams incident on the first surface and exiting thesecond surface, some of the light beams are reflected by the secondelectrodes, further improving efficiency of the solar cell.

The number of the second electrodes is equal to or more than the numberof the first electrodes.

In the instance when the number of the first electrodes is equal to thenumber of the second electrodes, two neighboring first electrodes of theplurality of first electrodes are spaced apart from each other by afirst pitch P1, and two neighboring second electrodes of the pluralityof second electrodes are spaced apart from each other by a second pitchP2, wherein the first pitch P1 is the same as the second pitch P1.

In the instance when the number of the second electrodes is more thanthe number of the first electrodes, two neighboring first electrodes ofthe plurality of first electrodes are spaced apart from each other by afirst pitch P1, and two neighboring second electrodes of the pluralityof second electrodes are spaced apart from each other by a second pitchP2, wherein the second pitch P2 is smaller than the first pitch P1.

According to an embodiment, a ratio n of the first pitch P1 and thesecond pitch P2 satisfies an equation: 0.1<n<1, where n=P2/P1).

Considering an area of a light receiving surface, the first pitch P1 islarger than the second pitch P2, and considering series resistances, thesecond pitch P2 is smaller than the first pitch P1. This increases theefficiency of the solar cell.

Each of the plurality of first electrodes has a line width of less than60 um and a thickness of more than 10 um. Each of the plurality ofsecond electrodes has a similar line width and thickness as each of theplurality of first electrodes. For example, each of the plurality ofsecond electrodes has a line width of less than 70 um and a thickness ofmore than 10 um.

The above structure may reduce a recombination loss of electricalcharges, thus further improving efficiency of the solar cell.

The substrate is of a first conductive type, wherein an emitter portionis provided on the first surface, wherein the emitter portion is of asecond conductive type opposite the first conductive type.

The plurality of first electrodes and the plurality of first currentcollectors contact the emitter portion. Accordingly, electric charges,for example, electrons or holes, attracted to the emitter portion aredirectly transferred to the plurality of first electrodes and the firstcurrent collectors, thus facilitating the flow of current.

A first insulating film is provided on a surface of the emitter portionat portions where the plurality of first electrodes and the plurality offirst current collectors are not provided. The first insulating film mayperform both an antireflection function and a passivation function.

An electric field portion is provided on the second surface, wherein theelectric field portion is of a first conductive type and having animpurity concentration that is higher than an impurity concentration ofthe substrate. A second insulating film is provided on a surface of theelectric field portion at portions where the plurality of secondelectrodes and the plurality of second current collectors are notprovided. The second insulating film may perform both an antireflectionfunction and a passivation function.

The plurality of second electrodes and the plurality of second currentcollectors contact the electric field portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view illustrating a portion of a solar cellaccording to an embodiment of the invention.

FIG. 2 depicts plan views of a first surface and a second surface of thesolar cell shown in FIG. 1 according to an embodiment of the invention.

FIG. 3 is a cross sectional view illustrating a portion of a solar cellaccording to another embodiment of the invention.

FIG. 4 depicts plan views of a first surface and a second surface of thesolar cell shown in FIG. 3 according to another embodiment of theinvention.

FIG. 5 is a graph illustrating efficiency of the solar cell shown inFIG. 3 depending on a ratio of a first pitch and a second pitch.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the invention will be described in more detail withreference to the accompanying drawings, wherein like reference numeralsmay be used to designate like or similar elements throughout thespecification and the drawings.

It will be understood that when an element such as a layer, film,region, or substrate is referred to as being “on” another element, itcan be directly on the other element or intervening elements may also bepresent.

In contrast, when an element is referred to as being “directly on”another element, there are no intervening elements present.

FIG. 1 is a cross sectional view illustrating a portion of a solar cellaccording to an embodiment of the invention. FIG. 2 depicts plan viewsof a first surface and a second surface of the solar cell shown in FIG.1 according to an embodiment of the invention.

The solar cell includes a substrate 110 that has first and secondsurfaces opposite to each other. The solar cell further includes anemitter portion 120, a first insulating layer 130, a plurality of firstelectrodes 140, and a plurality of first current collectors 150 on thefirst surface, and a back surface field (“BSF”) portion 160, a secondinsulating layer 170, a plurality of second electrodes 180, and aplurality of second current collectors 190 on the second surface.

The emitter portion 120 is disposed on the first surface of thesubstrate 110. The first insulating layer 130 is disposed on the emitterportion 120. The plurality of first electrodes 140 are disposed onportions of the emitter portion 120 where the first insulating layer 130is not arranged or has been penetrated by the first electrodes 140. Thefirst current collectors 150 are physically and electrically connectedto the first electrodes 140. In embodiments of the invention, the firstcurrent collectors 150 intersect (or cross) the first electrodes 140.The second insulating layer 170 is disposed on the BSF portion 160. Theplurality of second electrodes 180 are disposed on portions of the BSFportion 160 where the second insulating layer 170 is not arranged or hasbeen penetrated by the second electrodes 180. The second currentcollectors 190 are physically and electrically connected to the secondelectrodes 180. In embodiments of the invention, the second currentcollectors 190 intersect (or cross) the second electrodes 180.

The substrate 110 is formed of a semiconductor, such as a silicon waferhaving a first conductive type, for example, an n conductive type. Thesilicon wafer may include mono-crystalline silicon, polycrystallinesilicon, or amorphous silicon.

The substrate 110 having the n conductive type contains an impurity of aGroup V element, such as phosphorous (P), arsenic (As), or antimony(Sb).

According to another embodiment of the invention, the substrate 110 mayhave a p conductive type and/or may include a semiconductor materialother than silicon.

In the instance when the substrate 110 is of a p conductive type, thesubstrate 110 may contain an impurity of a Group III element, such asboron (B), gallium (Ga), or indium (In).

The substrate 110 has textured surfaces. Specifically, each of the firstand second surfaces of the substrate 110 is implemented as a texturedsurface.

The emitter portion 120 contains an impurity having a second conductivetype that is opposite to the first conductive type of the substrate 110.For example, the emitter portion 120 is of a p conductive type, andforms a PN junction with the substrate 110 when the substrate 110 is ofthe n conductive type.

Due to a built-in potential difference caused by the PN junction,electron-hole pairs generated by incident light are split into electronsand holes, and the electrons and holes are respectively moved toward ann type electrode and a p type electrode of the solar cell.

For example, if the substrate 110 is of an n conductive type, and theemitter portion 120 is of a p conductive type, then the electrons andholes are attracted to the substrate 110 and the emitter portion 120,respectively. Accordingly, the electrons for the substrate 110 and theholes for the emitter portion 120 become majority carriers.

The p conductive type emitter portion 120 may be formed by doping thesubstrate 110 with an impurity of a Group III element, such as B, Ga, orIn.

In the instance when the substrate 110 is of the p conductive type andthe emitter portion 120 is of the n conductive type, the holes areattracted to the substrate 110, and the electrons are attracted to theemitter portion 120.

The n conductive type emitter portion 120 may be formed by doping thesubstrate 110 with an impurity of a Group V element, such as P, As, orSb.

The first insulating layer 130 may be formed of at least one of asilicon nitride (SiNx:H) film, a silicon oxide (SiOx:H) film, and analuminum oxide (AlOx) film. The first insulating layer 130 may have bothan anti-reflection function and a passivation function. Reference tohydrogen (H) signifies that a film is hydrogenated or infused withhydrogen.

The first insulating layer 130 includes a plurality of openings thatexpose portions of the emitter portion 120. The plurality of firstelectrodes 140 are formed at the exposed portions of the emitter portion120 to contact the emitter portion 120.

The plurality of first electrodes 140 are each formed to have a firstthickness T1 of more than 10 um and a first line width W1 of less than60 um.

Although in FIG. 1 the thickness T1 is represented as a distance betweena top surface of the first electrode 140 and a protruded portion of theemitter portion 120, the thickness T1 may also be represented as (orrefer to) a distance between the top surface of the first electrode 140and a depressed portion of the emitter portion 120 since a distancebetween the protruded portion and the depressed portion is not greatcompared to the thickness of the first electrode 140.

Forming the first electrode 140 to have the line width and thickness maymaximize a light incident area of the first surface and may reducelosses caused by recombination of electric charges (e.g., holes andelectrons).

The first electrodes 140 may be formed by a printing process usingconductive paste or by an electroplating process.

The conductive paste may contain at least a conductive material selectedfrom the group consisting of Al, Ni, Cu, Ag, Sn, Zn, In, Ti, Au, and acombination thereof.

When formed by an electroplating process, the first electrodes 140 mayinclude a seed layer, a diffusion barrier layer, and a conductive layer.

The seed layer may be formed of a material containing nickel, forexample, nickel silicide including Ni₂Si, NiSi, or NiSi₂.

The diffusion barrier layer that is formed on the seed layer prevents orreduces a junction degradation from occurring due to diffusion causedwhen a material constituting the conductive layer spreads through theseed layer to a silicon interface.

The conductive layer is formed on the diffusion barrier layer. Theconductive layer includes at least a conductive metal, such as, forexample, Ni, Cu, Ag, Al, Sn, Zn, In, Ti, Au, or a combination thereof.However, the embodiments of the invention are not limited thereto, andother conductive metals may also be employed.

In the instance when the conductive layer includes a copper layer, a tinlayer may be further formed on the copper layer to prevent oxidation ofthe copper layer and to facilitate soldering of a ribbon.

The first electrodes 140 contact the emitter portion 120 through theopenings so that the first electrodes 140 are physically andelectrically connected to the emitter portion 120. As shown in FIG. 2,the plurality of first electrodes 140 are formed in a first direction(Y-Y′). A first pitch which is defined as a distance between twoneighboring first electrodes, is denoted as “P1”.

The first electrodes 140 collect electric charges, such as holes, at theemitter portion 120.

At least two first current collectors 150 are formed on a surface of theemitter portion 120 exposed through the openings in a second direction(X-X′) that crosses the first direction (Y-Y′).

The first current collectors 150 are physically and electricallyconnected to the first electrodes 140. The first current collectors 150contact the emitter portion 120. Accordingly, the first currentcollectors 150 collect electric charges attracted to the emitter portion120 and output the electric charges to an external device.

According to embodiments, the first current collectors 150 may be formedby electroplating, like the first electrodes 140, or by printing,drying, and firing conductive paste containing a conductive material.

The second electrodes 180 which are located on the second surface of thesubstrate 110 collect electric charges, for example electrons, movingtoward the substrate 110 and output the electric charges to an externaldevice.

As shown in FIG. 2, the second electrodes 180 are formed in the samedirection as the first electrodes 140, that is, in the first direction(Y-Y′), and are formed to have the same number as the number of thefirst electrodes 140. A second pitch P2 which is a distance between twoneighboring second electrodes 180 is identical to the first pitch P1.

The second electrodes 180 are positioned on different lines from thefirst electrodes 140 as shown in FIG. 1. The phrase “the secondelectrodes 180 are positioned on different lines from the firstelectrodes 140” refers t a central line CL1 of a first electrode 140 ina width direction does not conform to a central line CL2 of a secondelectrode 180 in a width direction. In embodiments of the invention, thedifferent lines may be referred to as reference lines, whereby thereference lines may be the center lines, but not necessarily. That is,the reference lines may be a line based on any common feature of thefirst electrodes 140 and the second electrodes 180, such as one of thelateral sides of each of the first and second electrodes 140 and 180.

According to an embodiment, the central line CL2 of the second electrode180 is apart from the central line CL1 of the first electrode 140 by apredetermined distance G.

Each second electrode 180 has a second line width W2 similar to thefirst line width W1 of the first electrode 140, for example, a linewidth of less than 70 um, and a second thickness T2 of more than 10 um.

Forming the second electrodes 180 to have the line width and thicknessas above may minimize resistance of the minority carriers and may reducelosses due to recombination of electric charges.

At least two second current collectors 190, which are physically andelectrically connected to the second electrodes 180, are formed on thesecond surface of the substrate 110. The second current collectors 190are formed in a direction crossing the second electrodes 180, that is,in the second direction (X-X′).

The second electrodes 180 and the second current collectors 190 may beformed of the same materials as the first electrodes 140 and the firstcurrent collectors 150, respectively.

The BSF portion 160 that contacts the second electrodes 180 and thesecond current collectors 190 and is physically and electricallyconnected to the second electrodes 180 and the second current collectors190, is formed on the overall (or the entire) second surface of thesubstrate 110. The BSF portion 160 is formed to have an area, forexample, an n+ area, doped with an impurity that is of the sameconductive type as the substrate 110 but with a higher concentrationthan a concentration of the substrate 110.

The BSF portion 160 forms a potential barrier based on a difference inconcentration of impurity between the BSF portion 160 and the substrate110, interfering with the movement of carriers, such as holes toward arear surface of the substrate 110. Accordingly, the electrons and holesare less likely to be recombined to perish near a surface of thesubstrate 110.

The second insulating layer 170 is arranged on a rear surface of the BSFportion 160 where the second electrodes 180 and the second currentcollectors 190 are not positioned. The second insulating layer 170 maybe formed of at least one of a silicon nitride (SiNx:H) film, a siliconoxide (SiOx:H) film, and an aluminum oxide (AlOx) film. The secondinsulating layer 170 may have both an anti-reflection function and apassivation function. Reference to hydrogen (H) signifies that a film ishydrogenated or infused with hydrogen.

The solar cell thusly configured may be used as a bifacial solar cell.An operation of the solar cell will now be described.

Due to light incident on the substrate 110 through the emitter portion120 and the BSF portion 160, electron-hole pairs are generated in thesolar cell.

Since the first and second surfaces of the substrate 110 are formed astextured surfaces, escape of light by reflection is reduced at the firstand second surfaces, and light incidence and reflection occur at thetextured surfaces, so that light is trapped inside the solar cell.Accordingly, light absorption increases, thus improving efficiency ofthe solar cell.

Further, a loss due to escape of incident light by reflection off thesubstrate 110 is decreased by the first insulating layer 130 and thesecond insulating layer 170, thereby increasing the amount of lightentering the substrate 110.

When generated by the incident light, electron-hole pairs are separatedfrom each other by the PN junction of the substrate 110 and the emitterportion 120, so that the electrons are attracted to the substrate 110which is of an n conductive type and holes are attracted to the emitterportion 120 which is of a p conductive type.

Then, the electrons are moved from the substrate 110 to the secondelectrodes 180 and the second current collectors 190 via the BSF portion160, and the holes are moved from the emitter portion 120 to the firstelectrodes 140 and the first current collectors 150.

Accordingly, when the first current collectors 150 are connected to thesecond current collectors 190 through a conductive line, such as aninterconnector, then current flows through the conductive line.

Since the second electrodes 180 are positioned on different lines fromthe first electrodes 140, some of light beams that are incident on thefirst surface, and which pass through the substrate 110 and the secondsurface to the outside, are reflected by the second electrodes 180 andreenter the substrate 110.

Likewise, among light beams incident on the second surface, some of thelight beams that pass through the substrate 110 and exit the firstsurface are reflected by the first electrodes 140 and reenter thesubstrate 110.

Accordingly, the amount of light that contributes to photoelectricconversion increases, thus improving efficiency of the solar cell.

FIG. 3 is a cross sectional view illustrating a portion of a solar cellaccording to another embodiment of the invention. FIG. 4 depicts planviews of a first surface and a second surface of the solar cell shown inFIG. 3 according to another embodiment of the invention. FIG. 5 is agraph illustrating efficiency of the solar cell shown in FIG. 3depending on a ratio of a first pitch and a second pitch.

The basic configuration of the solar cell according to this embodimentof the invention is the same or substantially the same as the solar celldescribed in connection with FIGS. 1 and 2, wherein the same referencenumbers may be used to denote the same or substantially the sameelements as those described in connection with FIGS. 1 and 2.

The substrate 110, the emitter portion 120, the first insulating layer130, the first electrodes 140, the first current collectors 150, the BSFportion 160, and the second insulating layer 170 are the same orsubstantially the same as those described in connection with FIGS. 1 and2.

A difference of this embodiment from the embodiment described inconnection with FIGS. 1 and 2 is that the number of the secondelectrodes 180 is more than the number of the first electrodes 140.

The second electrodes 180 are formed so that two neighboring secondelectrodes 180 are spaced apart from each other by a second pitch P2,and the first electrodes 140 are formed so that two neighboring firstelectrodes 140 are spaced apart from each other by a first pitch P1,wherein the second pitch P2 is smaller than the first pitch P1. Thesecond electrodes 180 are arranged on different lines from the firstelectrodes 140.

Considering an area of a light receiving surface, the first pitch P1 islarger than the second pitch P2, and considering series resistances, thesecond pitch P2 is smaller than the first pitch P1.

Compared to the embodiment described in connection with FIGS. 1 and 2,in this embodiment, there is a great number of the second electrodes 180that may reflect light passing through the substrate 110 back to thesubstrate 110, further enhancing efficiency of the solar cell.

In the embodiment of the invention of FIGS. 1 and 2, the firstelectrodes 140 and the second electrodes 180 are not aligned. In theembodiment of the invention of FIGS. 3 and 4, none of the firstelectrodes 140 may be aligned with any of the second electrodes 180, orat least some of the first electrodes 140 may be aligned with some ofthe second electrodes 180. Additionally, in the embodiments of theinvention, one of none, some or all of the first current collectors 150may be aligned with the second current collectors 190.

FIG. 5 illustrates a variation in efficiency depending on a ratio n ofthe first pitch P1 and the second pitch P2 (that is, n=P2/P1), whereinthe horizontal axis refers to the ratio n and the vertical axis refersto the efficiency (%).

Referring to FIG. 5, it can be seen that the solar cell has the maximumefficiency when the ratio n of the first pitch P1 and the second pitchP2 is more than 0.1 and less than 1, that is, when the ratio n satisfiesthe following equation: 0.1<n<1.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the scope of the principles of thisdisclosure. More particularly, various variations and modifications arepossible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A bifacial solar cell comprising: a substrate; anemitter portion formed on a first surface of the substrate; a firstinsulating layer formed on the emitter portion; a plurality of firstelectrodes contacting the emitter portion through the first insulatinglayer and extended in a first direction; a plurality of first currentcollectors extended in a second direction crossing the first direction,wherein the plurality of first current collectors are electrically andphysically connected to the plurality of first electrodes; a secondinsulating layer formed on a second surface of the substrate; a backsurface field formed on the second surface of the substrate, and havingan impurity concentration that is higher than an impurity concentrationof the substrate; a plurality of second electrodes contacting the backsurface field through the second insulating layer and extended in thefirst direction; and a plurality of second current collectors extendedin the second direction, wherein the plurality of second currentcollectors are electrically and physically connected to the plurality ofsecond electrodes, wherein a number of the plurality of secondelectrodes is different from a number of the plurality of firstelectrodes, wherein two neighboring first electrodes among the pluralityof first electrodes are spaced apart from each other by a first pitch P1, and two neighboring second electrodes among the plurality of secondelectrodes are spaced apart from each other by a second pitch P2, andwherein the second pitch P2 is smaller than the first pitch P1.
 2. Thebifacial solar cell of claim 1, wherein a central line of each of theplurality of second electrodes in a width direction is positioned not toconform to a central line of each of the plurality of first electrodesin the width direction.
 3. The bifacial solar cell of claim 1, wherein aratio n of the first pitch P1 and the second pitch P2 satisfies anequation: 0.1<n<1, where n=P2/P1.
 4. The bifacial solar cell of claim 1,wherein each of the plurality of first electrodes has a line width ofless than 60 μm and a thickness of at least 10 μm.
 5. The bifacial solarcell of claim 1, wherein each of the plurality of second electrodes hasa line width of less than 70 μm and a thickness of at least 10 μm. 6.The bifacial solar cell of claim 1, wherein the substrate is formed of nconductive type silicon, and the emitter portion is of a p conductivetype.
 7. The bifacial solar cell of claim 6, wherein the emitter portionis formed of a p conductive type silicon.
 8. The bifacial solar cell ofclaim 1, wherein the first insulating layer includes at least one of asilicon nitride layer, a silicon oxide layer, or an aluminum oxidelayer.
 9. The bifacial solar cell of claim 1, wherein the secondinsulating layer includes at least one of a silicon nitride layer, asilicon oxide layer, or an aluminum oxide layer.
 10. The bifacial solarcell of claim 1, wherein two neighboring second electrodes among theplurality of second electrodes are aligned within a spacing of twoneighboring first electrodes among the plurality of first electrodeswhen viewing a side view of the bifacial solar cell.
 11. The bifacialsolar cell of claim 1, wherein the plurality of first electrodesdirectly contact the emitter portion through openings of the firstinsulating layer.
 12. The bifacial solar cell of claim 1, wherein theplurality of second electrodes directly contact the back surface fieldthrough openings of the second insulating layer.
 13. The bifacial solarcell of claim 1, wherein the substrate is a silicon substrate.